What power are we talking about? That's completely new to me (and I had to go through this process).

Just to give an example of a document that I know had been submitted in this situation. A graduation certificate from a Jewish girls gymnasium in Vilno. The city has changed name since then, there's no such street address, there aren't any girl schools, definitely not gymnasiums, let alone Jewish. The building that used to be the school was destroyed in WW2. So, there's nobody who can vouch for the document. Maaaybe you could somehow find an index of all such schools that exited in the year of graduation, but even this info might not be available.

During WW2 a lot of civil records have been lost, especially in smaller towns / villages. Sometimes it was deliberate, especially if it was a Jewish settlement. It was common for Jews in the military to try to erase any trace of their ancestry, as regardless of how poorly the Red Army PoWs were treated, Jews and Communists would've been executed immediately. So, destroying records indicating such connections and forging personal documents was a common case. Now that people try to recover any traces tying them to their ancestors, they often have very little to rely on. Like, receipts from donating to a synagogue, or permits to start a particular business (typically associated with being a Jew) etc.

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Another funny memory I have in this respect: in the 90's I was queuing in a bakery in some central part of Lviv. A man behind me overhead me speaking Russian (which wasn't very common at the time, since Lviv citizens frowned upon it, and mostly spoke Ukrainian), and decided to ask me if I know where Adolf Hitler street was.

My jaw dropped. But, the man pulled out from a pocket a triangular letter (the kind soldiers used to send during the war) with the address specifying exactly that. Apparently, the carrier of the letter was looking for his long-lost friend whose last known address was in Lviv, on that unfortunately named street. And since Lviv was seen as being quite radical in their way to dedicate streets to questionable historical figures, the old man believed that they might just have such a street...

Anyways, some locals overheard our conversation, and soon we discovered that the street in question was indeed named after Hitler during the German occupation, after Soviet occupation was renamed the Lenin street, but historically was called Lychakivska (and that was its current name, restored in the recent years).

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Another similar story involves my dad's friend who was born in the 30's when the Soviets and the Nazis had a love affair. So, this guy was named Adolf, yes you guessed it, after the Austrian painter. He was Jewish. So, after the love affair ended, he sought to change his name. But you cannot change the name on the birth certificate. Also, his school graduation papers etc. all had him as Adolf, and that's how his family called him. Sort of. (I knew him as "uncle Dolik".) Not surprisingly, there wasn't much of a record of him changing his name to Alexei :) and he'd routinely get in trouble with all kinds of authorities, police when checking his driver's license, paying electricity bills etc.

Similarly, in Western Ukraine, prior to Soviet occupation, it was customary to give two names to children. Eg. my grandmother was Daria Anna. But the Soviet system didn't acknowledge this, and only one name could go into the passport / city records. So, she became Daria. At first. Then Dariana. And after having all sorts of documents, she was in a very tough spot proving ownership of her apartment, because it wasn't possible to tell (from the authorities perspective) whether Daria Anna, Daria and Dariana were the same person. Add to this that in order to preserve some of the family property she and her remaining relatives tried to mud the waters around these documents. Eg. to avoid partitioning the apartment she'd claim to have a sister Anna, who lived at the same address...

I'm quite sure that this wasn't an isolated incident. There would've been a lot of attempts to manipulate the system by creating fake people, trying to wipe out one's own records etc. Paper documents help in detective work to untangle such manipulations. If there was ever a single central source of this information, such manipulations would've been a lot more successful.